Despite a long standing awareness that both photic stimulaton and gonadal hormones control pituitary gonadotropin release, the capacity of these two factors to interact and thereby regulate the hypothalamo-hypophyseal gonadal axis in mammals is largely unknown. I have completed preliminary experiments that indicate in the male golden hamster these two factors act together to regulate pituitary gonadotropin secretion. I have demonstrated that the post-castrational increase in pituitary gonadotropin release is abolished by depriving male hamsters of photoperiodic stimulation. In the present proposal, I propose to study the dual effects of photic stimuli and castration on the hypothalamic-hypophyseal system regulating pituitary gonadotropin secretion. I plan to assess the relationships between the endocrinological and physiological state of the organism and the time at which castration fails or begins to influence the neuroendocrine system regulating gonadotropin secretion. Once this is established, I propose to investigate the role of photic stimuli in altering the ability of the pituitary gland of castrate male hamsters to release gonadotropins in response to exogenous testosterone or to a hypothalamic gonadotropin releasing factor. Experiments have been proposed to assess the involvement of the pineal gland in altering the response of the hypothalamic-pituitary system to the effects of castration during an exposure to short photoperiods. By integrating the information obtained from the above studies, I hope to develop a coherent story as to how photic stimuli and gonadal hormones act together as well as separately in regulating pituitary gonadotropin release.